Where and how to grow medicinal plants requires a keen eye, patience and plenty of experience

The Canadian consumer demand for medicinal plants is booming and producers are trying to catch up (Arnason, 2015). A 2016 survey done by the Fraser Institute found that about 79% of Canadians had used plant-based therapies at some point in their lives, including herbal extracts, aromatherapy, and naturopathy; with just over 25% of the population using them within the 2016 year (Esmail, 2017). Some estimate that the value of the domestic medicinal plant industry is worth about $300 million per year. There is great potential for industry growth in Canada, because there are over 4,000 identified medicinal plants that can grow in the many Canadian regions, both wild and cultivated (Westerveld, 2012). Canadian herbalists and producers prefer domestically grown plants to supply local and international customers but producing high quality medicinal plants is not easy (Arnason, 2015). Within this budding industry, expertise on when, where and how to grow and harvest requires a keen eye, patience, and plenty of experience which is often gained by applying science and many sources of traditional knowledge.

Figure 1: Harvesting medicinal plants requires scientific and traditional knowledge. Thyme is harvested before flower buds bloom. Photo by Evan Amos, wikimedia
Figure 1: Harvesting medicinal plants requires scientific and traditional knowledge. Thyme is harvested before flower buds bloom. Photo by Evan Amos, Wikimedia

What defines quality? Quality of medicinal plants does not relate to how big the plant is (its biomass) nor how nice it looks (its aesthetics), which are important characteristics in food plant production because consumers buy food based on appearance (Westerveld, 2012; Novák & Németh, 2002). In contrast, levels and consistency of desired biochemicals define the quality of plant medicine. Plants contain mixtures of compounds that change over time as plants grow and cope with a changing environment. For example, temperature, amount of rain, soil type, presence of pests, and sunlight can influence the content of important biochemicals (Ncube et al., 2012). Different plants are used for different ailments and each species requires detailed knowledge from the part of the picker to deliver optimum health benefits. Further to knowing each plant separately and intimately, the World Health Organization has written international harvesting guidelines and plant conservation standards for optimal harvest times which consider the time of day, season, stage of plant development (buds or fully developed parts), frequency of collection (year-to-year, or every other year, etc.), and plant part picked (leaves, roots, flowers) (Leaman, 2008).

Influence of the environment on optimal harvest time

Plants cannot move and thus remain in one spot throughout their lives, so they have evolved sophisticated chemical defense mechanisms to provide protection against some of the dangers in the environment (Ncube et al., 2012). The protective chemical compounds make up what biochemists called secondary metabolites. It is inevitable that environmental variations be present that can affect individual plants differently, thus causing variation in chemical composition. Two different plants of the same species can present different amounts of medicine, depending on a multitude of factors. For example, seasonal temperature and changes in daylight hours, or weather fluctuations like droughts and frosts can affect the strength and ability of a plant to produce desired medicines (Selmar & Kleinwachter, 2013). Throughout the growing season, plants face stresses such as ultraviolet light, which can cause damage. Some compounds can act as sunscreens and selectively block damaging light while allowing visible light to pass through the plant tissues so the plant can survive (Ncube et al., 2012). High or low temperatures can increase or decrease plant production of chemicals that act as antioxidants, provide desirable scents or vitaminshighly valued to promote well-being(Ncube et al., 2012). The ongoing environmental conditions in which one species grows and when harvest happens, can impact both medicinal compound abundance and ease of management (Archarya et al., 2010). The following examples illustrate the many conditions affecting harvest time in several medicinal plants grown in Canada.

American mayapple: sun or shade?

Variations in sunlight can affect quality within a species. Cushman et al. (2006a; b) studied two populations of American mayapple (Podophyllum peltatum), growing wild in either sunny or shaded terrain. The plants were harvested twice, once early in the season and once late in the season. They measured the amount of podophyllotoxin or podofilox, a powerful compound used to treat the herpes virus in humans. For the sun-grown population, early harvests gave about two to six times more medicine than late harvest. Meanwhile, the shaded population showed no significant changes in content across time but the amount of podofilox produced was much lower than in sun-grown plants. So, sun-grown plants provide more medicine even if they can only be harvested early in the season.

Figure 2: Mayapple bloom. (Photo: WilderAddict, CC BY-SA 4.0, via Wikimedia Commons)

German chamomile: noon or dusk?

Salehi and Hazrati (2017) analyzed essential oil content of German chamomile (Matricaria chamomilla) flowers throughout the day. Flowers were collected at two-hour periods from 8 am to 8 pm. The plants with the greatest quantity of oils were picked before noon from 10:00-12:00, while the lowest were from 6:00 to 8:00 pm, so hour of harvest is an important consideration. These scientists also mention temperature and relative humidity as factors to consider in addition to sunlight intensity. They speculated that when measuring chemical content of the chamomile flower alone, levels are higher in the morning because warmer, drier, sunnier conditions later in the day promote production of other protective compounds needed by the plants at the expense of the oils which are desired as anti-inflammatories in humans.

Influence of plant physiology on medicinal potential

Peppermint: blooming or not blooming?

Zheljazkov et al. (2010) measured levels of the most abundant constituents present in essential oil at two developmental stages of the peppermint plant (Mentha piperita). The study looked at menthol and menthone levels, the two main components of medicinal value. The authors analysed all plant parts during the bud formation stage and also during full bloom. Menthol concentrations was higher in the buds than in fully developed flowers. Marcum & Hansen (2006) also discovered that menthol concentrations increased throughout the season, but so did levels of menthofuran, a bitter compound that is undesirable and reduces the value of peppermint oil as a digestive aid. Depending on the limits of menthofuran accepted by processors, growers tend to choose to harvest before maximum levels of menthol are present to avoid menthofuran as much as possible. Harvesters must familiarize their senses with the many signs of plant development, while using a good dose of patience.

Horse chestnut: young seeds or mature seeds?

Another example of the chemical balancing act to optimize medicinal plant properties is horse chestnut (Aesculus hypocastanum). Horse chestnut composition also varies during seed development. A compound of interest is escin, which is used as an anti-inflammatory in cardiovascular diseases. In a study done by Kedzierski et al. (2016), the seeds were collected every two or three weeks after flowering over a period of 21 weeks.

Figure 3: Horse chestnut seeds. (Photo by Solipsist~commonswiki, CC BY-SA 2.0, Creative Commons)

Escin was not present before week 7 and remained minimal until week 12 when it began to increase, reaching a peak content at week 19 before declining slightly at week 21, so medicinal content in the seeds changed over time giving a window of opportunity of less than two weeks to harvest. In horse chestnut, there are other medicines besides escin, that are produced separately so harvesting decisions depend on the presence of more than one compound. By harvesting the seeds at the overlap when several compounds are relatively high (week 14-15) medicinal content can be optimized.

Thyme and pennyroyal: the young buds or the mature flowers?

Zantar et al. (2015) analyzed the yield of antimicrobial essential oils extracted from leaves and flowers of garden thyme (Thymus vulgaris) and pennyroyal (Mentha pulegium) at pre-flowering and full bloom stages. Garden thyme showed maximum overall quality at full bloom, while pennyroyal quality was highest at pre-flower. So, decisions on when to harvest to obtain higher essential oil yields are based on which part of the plant is picked in different species. Harvest timing recommendations must be made individually for each species because generalizing trends leads to lower quality.

Figure 4: Pennyroyal buds. (Photo by Eiku, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Common)

Echinacea: Spring or late Summer?

Thomsen et al. (2012) analyzed how the medicinal compounds in echinacea (Echinacea purpurea and Echinacea pallida) roots change over a season of growth. Spring collection (March) showed the highest concentration of phenolic compounds, which are desired as immunity boosters, while collection in August showed the lowest concentration. Levels peaked when the shoots sprouted or were at highest growth rate. Important to note is that this information directly contradicts existing recommendations proposing fall harvest among some traditional herbalists. This highlights a lack of consensus about harvesting time of these popular plants.

Labrador Tea: the Innuit know best

Black et al. (2018) studied the levels of 15 compounds through the entire growth cycle of Labrador Tea (Rhododendron tomentosum subarticum) in Iqaluit, during the snow-free summer season from June to September. These compounds benefit the digestive tract when considered together and had maximum concentration twice in the season: late June/early July and again in mid/late September.

Figure 5 – Labrador Tea (Photo Credit: Allen Stegeman)

The early season peak corresponds to when buds are forming. This may be because flowers are valuable to the plant as reproductive organs and they need to be protected from disease; it make sense that defensive chemicals are allocated to the buds and away from the leaves. The late season peak corresponds to the leaf death stage. Black et al. (2018) do not suggest any explanation for these levels late in the season, so further research is needed, but the Innuit communities traditionally harvest for Labrador Tea twice per season, which coincides with current scientific evidence.

Concluding remarks

We need to know more about harvesting to strengthen the Canadian medicinal plant industry. To harvest a plant at its peak of medicinal benefit depends on the juggling between environment, the plant’s unique biochemistry and stage of growth. Harvesting for medicinal plants is therefore as much a science as it is an art where knowledge is gained through experience. Harvesting generalizations do not apply to all plants. An in-depth understanding of each plant species is essential to an efficient medicinal plant enterprise. For mass production, more detailed research about what affects the medicinal content is needed, but traditional knowledge holders who have valuable experience and the legacy of past generations are invaluable contributors.

About the Authors

Heather Beach has a Bachelor of Science in Agriculture (Organic Agriculture Major), ’19, from the University of Guelph. Besides learning about and participating in ecological farming, she enjoys nature hikes, reading fiction and cooking with plants.


Spencer McGregor is a recent graduate from the University of Guelph. Having received his degree in Organic Agriculture in June 2019, Spencer is now working on an organic vegetable farm in Guelph, Ontario as an intern to gain more practical farming experience.


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Posted by Shweta Dixit